This invention relates in general to telecentric laser printers for microfilm and in particular to high spot density telecentric laser printers using overfilled polygon scanners.
Most laser printers using polygonal mirrors as the scanner for the laser beam use under-filled designs. This means that the laser beams are smaller than each mirror facet of the polygon rotating through the beam incident on it. As the requirements for very high pixel densities at the film increase to very high values such as 8000 dots per inch or more, these polygons necessarily become larger is size. Large diameter polygons are more expensive to manufacture with high quality facets and the increased weights place heavier demands on the motor drives.
Polygon scanners rotate across the incident beam causing, in effect, a variation of the entrance pupil of the F-theta scan lens focusing the beam onto the film because the reflection point of the beam moves. Larger diameter polygons produce proportionally more motion of the effective entrance pupil for the scan lens. This leads to a larger polygon induced field curvature different in both the scan section of the beam (the section of the beam parallel to the scan line) and in the cross-scan section of the beam (the section of the beam perpendicular to the scan line). These polygon induced field curvatures must be compensated to a very high degree by the field curvatures of the F-theta scan optics in order to keep the pixel forming spot in the same focal plane as it scans across the film. This problem is compounded in very high pixel density systems due to the inherently shallow depth of focus of a very small pixel. Therefore achieving high pixel densities with polygon scanners places heavy demands on the F-theta lens leading to complex designs.
Examples of the complexities of computer output microfilm (COM) F-theta lenses are given in U.S. Pat. No. 4,880,299, none of which have fewer than six rotationally symmetric elements in addition to anamorphic optics to compensate for polygon wobble and facet pyramid error. The requirement of telecentricity in such laser scanners adds to the complexity and size since the diameter of these lenses cannot be less that the total length scanned on the film.
In addition, while microfilm F-theta systems do not typically require color correction across the complete visible spectrum, they do need to have a certain correction for small variations in laser wavelength from laser to laser, or for mode hops as disclosed by Takanashi in U.S. Pat. No. 5,247,385. Some laser printers are designed for multiple mode lasers as disclosed by Isizuka in U.S. Pat. No. 4,863,250, but this is also a broader spectrum than for single mode laser as disclosed in this invention. Some other broad spectral range corrected telecentric F-theta lenses are disclosed by Cobb in U.S. Pat. No. 5,404,247 and Simbal in U.S. Pat. No. 5,087,987.
Objects of this invention are to reduce the polygon induced field curvature through the use of an overfilled polygon, to provide well balanced field curvatures across a 33 mm scan length for pixel densities over 7900 dots per inch, specific chromatic correction over a narrow wavelength range about 685 nanometers, and a well corrected cross-scan optical system for polygon pyramid error.
Briefly according to one aspect of the present invention a laser microfilm printer having a telecentric F-theta scan lens with optical resolution for image pixel sizes 1-5 microns for writing on an image-receiving medium comprises a polygon reflector. A laser beam incident on the polygon reflector, has a diameter as measured at the 1/e2 intensity value, which is at least 1.4 times a width of a polygon facet in a scan section of the laser microfilm printer. The polygon facet, at center of scan position, is in a substantially optically conjugate relationship with the image-receiving medium in a cross-scan section of the laser beam. The cross-scan section of the F-theta lens has an absolute value of optical magnification of 0.25 to 0.5 as measured from the facet to the image-receiving medium.
The subject of this invention differs from laser computer output microfilm (COM) printers in that COM printers expose in a bi-tonal fashion whereas this invention is a multilevel gray scale exposure printer.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.